rates of reactions

Question 1

What is the rate equation

Answer 1

Rate = k [A]^x[B]^y , where A and B are reactants

Question 2

What are the units for rate

Answer 2

Usually moldm-3 however can be units on y axis / units on x axis -1

Question 3

What is an order of reaction

Answer 3

In a reaction , different reactants have different orders and each will effect the rate of reaction differently . Zero order will not have an effect on rate , 1 st order will have a proportional effect on rate , and 2nd order will have a squared effect on rate

Question 4

How do you state what order a reactant is

Answer 4

• a reaction is ………. Order with respect to [X]

Question 5

How do you work out the overall order of a reaction

Answer 5

Add the powers

Question 6

What are the different ways rate can be measured

Answer 6

• change in PH of a reaction , eg if H+ ions are used up or produced
  -amount of mass lost (gas being produced)
  -volume of gas produced
  -colorimeter - measures the absorbance of light , the more concentrated , the more pigment and the more light absorbed
  -

Question 7

What is the rate constant

Answer 7

The rate constant K , allows us to equate rate and concentration

Question 8

What happens with K

Answer 8

k is only fixed at a particular temperature , if temperature changes so does the rate constant . K increases when temperature increases . The larger the value of ‘k’ the faster the rate of reaction
Because particles have more kinetic energy and frequency of collisions increases
- concentrations of substances remain constant so k must change .

Question 9

How do you find the units and value for k

Answer 9

• through experimental data find the rate equation
• rearrange for k and sub in units
• use experimental data to sub in values to find the rate constant

Question 10

how do you calculate Ea from a graph

Answer 10

• first we find the 1/T values and the lnk values and plot 1/T(x) against lnk to find the gradient . the gradient we then x 8.314 to give the activation energy in joules
  then we /1000 to get the activation energy in kj

Question 11

what is the defintion for an intermediate

Answer 11

a species formed in one step of a multi-step reaction that is used up in a subsequent step , and is not seen as either a reactant or product of the overall reaction

Question 12

what is the rate determining step

Answer 12

the slowest step in a reaction mechanism

Question 13

what does the overall rate equation equal

Answer 13

the rate of the slowest step

Question 14

what do you include in the rate equation

Answer 14

• the rate determining step reactants , and any reactants before this

Question 15

what does the order with respect to a particular reactant also tell us

Answer 15

the number of molecules of that reactant that participate in the rate determining step

Question 16

what is the Arrhenius equation

Answer 16

an expression that describes how the rate constant (k) for a given reaction varies with temperature and activation energy

k=Ae ^ -Ea/RT

k is the rate constant
A is the pre - exponential factor
EA is the activation energy in joules
R is the ideal gas constant
T is the temperature

Question 17

what can we do with this equation

Answer 17

we can log both sides (ln) which allows us to plot a graph and find the activation energy

Question 18

what does A mean

Answer 18

A takes into account the frequency of collisions with the correct orientation